At the end of the 19th century, the Swedish Nobel laureate Svante Arrhenius suggested that a doubling in atmospheric carbon dioxide (from coal burning) would increase the Earth's mean temperature by 5-6 degrees Celsius. Numerous climate models have been developed in recent decades to study in more detail the atmosphere's response to such human-induced perturbations. Though early models focused entirely on CO₂, in the last 10-15 years climate researchers have recognized the importance of such chemically and radiatively active gases as methane, nitrous oxide, halocarbons and ozone. Some of these are produced by the biosphere, but their concentrations are all perturbed on the global scale by human activities. These greenhouse gases contribute to global warming.

Now a variety of aerosols are being recognized for their impact on climate. Sulfate aerosol particles, produced through coal burning, scatter back to space a significant fraction of incoming solar radiation, leading to potential regional cooling in industrialized/urbanized areas. In this issue of IGACtivities, Barry Huebert and Don Lenschow discuss several field campaigns designed to investigate the chemical and microphysical processes involved in the formation and fate of these aerosols. Because aerosol particles are believed to serve as condensation nuclei for the formation of clouds, aerosols of anthropogenic origin are expected to produce an indirect climate forcing through changes in the optical properties of the clouds. This difficult scientific issue is discussed in the following pages by John Seinfeld and Richard Flagan.

Other types of aerosols are also significant. Black carbon (soot) tends to absorb a fraction of the solar and terrestrial radiation, thus warming the atmosphere. Irina Sokolik describes in this issue how mineral dust affects the atmosphere's radiative balance and hence the climate system. Sea salt and organic aerosols are additional particles that need to be considered in comprehensive climate studies.

One of IGAC's foci is atmospheric aerosols. The importance of this issue for global change is becoming increasingly evident. Aerosols not only affect climate, they play an important role in the chemistry of the stratosphere and troposphere through a number of heterogeneous reactions, and have a significant impact on the intensity of photolytic radiation. They affect visibility, damage plants, and can be detrimental to health. But, while fouling the air, aerosols also create beautiful sunsets!